Simultaneous production of heat and steam by combustion of a so-called bio-fuel, i.e., a solid fuel consisting of wood or biomass, has lately become more and more common, inter alia due to the facts that such a production is power-efficient, shows endurance in the long run, may be based on domestic raw materials and gives a minimal influence on the environment. However, it has turned out that the combustion of bio-fuel is a process that in some respects is more complicated and difficult to handle than the combustion of other solid fuels, such as coal. One complication is that the ashes from a bio-fuel has another composition and other melting properties than, e.g., coal ash. Inter alia, this difference involves costly problems with corrosion and ash deposition on the tubes included in existing superheating plants. Thus, serious high temperature corrosion has been observed in the major part of combined power and heating plants in Sweden after some years of operation with 100% bio-fuel. The problems may become particularly accentuated when to the fuel are added such materials as demolition timber and sorted waste of different types. In practice, the corrosion manifests itself in that the usually high-alloyed, and thereby expensive, superheater tubes are coated with stout, strongly adhering layers or deposits of ash, at the same time as the tube surface underneath is exposed to corrosive melts which give rise to a loss of metal.
Among experts, unanimity reigns that chlorine constitutes the main corrosion accelerator in the above-mentioned context. A conventional theory is that chlorine is transported into the ash deposit on the superheater tubes in the form of gas phase potassium chloride (KCl), alternatively as very small aerosols of potassium chloride that have condensed immediately upstream of the superheater device. Thereafter, a reaction with sulphur takes place on the tube surface in the ash deposit, thereby forming potassium sulphate and free chlorine, which in this form is very corrosive. Albeit this theory is plausible, in practice great difficulties exist not only to verify this theory but also to take measures to solve the problem, above all due to the lack of a suitable measuring technique. It is true that in SE 8502946-0, it is in general terms described how photo-spectrometry may be utilized to determine certain parameters, e.g., the concentration, for gaseous substances that occur in such combustion processes that are performed at high temperatures, but in this case the technique is primarily focussed on measuring in flames, and the document does not contain any instructions as to how the technique would, in practice, be used for measurements in plants of the type presented in the preamble.
Quite generally, in heat-producing plants occur, besides the above-mentioned corrosion problems, also other similar problems that are caused by the presence of gaseous metal chlorides or metals in elementary form. Hence, in the plants may be included also other arrangements than merely superheater devices comprising sets or packages of tubes, through which for instance air is circulated in order to be heated (in practice, such arrangements usually consist of air pre-heaters or so-called economizers). When metals, such as heavy metals in the form of zinc and lead in gaseous form, are carried by the flue gases and hit the arrangements, they are deposited on the surfaces of the tubes, thereby forming deposits that are not necessarily corrosive, but that deteriorate the heat transfer from the flue gases to the medium that circulates within the tubes.